1) Field of the Invention
The invention relates to the use of protective colloid-stabilized vinylaromatic-1,3-diene copolymers in the form of their aqueous polymer dispersions or polymer powders which are redispersible in water for the modification of gypsum-based materials or of materials based on calcium carbonate.
2) Background Art
Gypsum is a building material which is available in large quantities at low cost. Apart from naturally occurring forms of gypsum, considerable amounts of gypsum are obtained from flue gas desulfurization plants and there is worldwide interest in utilizing these. Apart from the use of gypsum in jointing compositions, gypsum mortars are used, in particular, for interior plasters and coatings. Since gypsum cures with an increase in volume, it is the ideal binder for coatings and moldings in order to avoid possible crack formation. However, owing to their water-sensitivity which is reflected, inter alia, in an unsatisfactory freezing/thawing behavior, renders comprising plaster of Paris as binder, with or without proportions of builder""s lime, may, according to DIN standard 18550, be used only for interior plasters subject to stresses customary for these, but not for damp rooms or for external renders. In order to be able to use gypsum-based building materials for external applications and wet applications, too, they have to be sufficiently hydrophobicized.
In DE-A 3704439 (U.S. Pat. No. 4,851,047), the use of silicones and siloxanes, steareates and paraffin waxes is proposed for hydrophobicizing gypsum mortars. EP-A 320982 describes the use of redispersion powders based on vinyl acetate-Versatic acid-vinyl ester copolymers for hydrophobicizing gypsum-based materials. EP-A 477900 discloses the use of dispersible powder compositions based on vinyl ester polymers or styrene-acrylate polymers as additives for improving mechanical properties such as adhesion, abrasion resistance and flexural strength in gypsum, building adhesives and mortars. EP-A 728715 recommends the use of compositions of dispersible powders and thixotropic additives for hydrophobicizing gypsum, with the dispersible powders recommended being ones based on vinyl acetate copolymers and on styrene-acrylate copolymers.
JP-A 5/836 (Derwent Abstract AN 93-49422) describes the hydrophobicization of cement and gypsum by means of a pulverulent mixture of polysiloxane and a copolymer of vinylaromatics, dienes and/or acrylates. JP-A 57/205352 (Derwent Abstract AN 83-10505K) describes the production of water-resistant gypsum moldings by addition of polymer latices of acrylate, styrene, vinyl ester and epoxy resins to the gypsum mortar and subsequent heat treatment to cure the moldings.
A disadvantage of the previously mentioned hydrophobicizing agents is their not negligible hydrophilicity, for example in the case of vinyl ester or acrylic ester polymers, which leads to a low water resistance. In the abovementioned Japanese publications, the emulsifier content of the latices employed leads to increased water absorption. This can be improved by the addition of additives such as siloxanes, but only within certain limits. In many cases, the mechanical strength obtained in the hydrophobicization is also unsatisfactory.
It is therefore an object of the invention to provide a composition for the hydrophobicization of gypsum-based materials and of materials based on calcium carbonate, which leads to hydrophobicized materials having a high water resistance and high mechanical strength without further additives.
The invention provides for the use of protective colloid-stabilized vinylaromatic-1,3-diene copolymers in the form of their aqueous polymer dispersions or polymer powders which are redispersible in water for the modification of gypsum-based materials or of materials based on calcium carbonate, wherein the polymer dispersions or polymer powders are obtained by emulsion polymerization of a mixture comprising at least one vinylaromatic and at least one 1,3-diene, in the presence of one or more protective colloids and in the absence of emulsifiers, and, if desired, drying of the aqueous polymer dispersions obtained in this way.
Suitable vinylaromatics are styrene and methylstyrene; preference is given to copolymerizing styrene. Examples of 1,3-dienes are 1,3-butadiene and isoprene; preference is given to 1,3-butadiene. In general, the copolymers comprise from 20 to 80% by weight, preferably from 30 to 70% by weight, of vinylaromatic and from 20 to 80% by weight, preferably from 30 to 70% by weight, of 1,3-diene. Further monomers may also be present if desired, and the percentages quoted in each case add up to 100% by weight.
If desired, up to 30% by weight, based on the total weight of the monomer phase, of further monomers which can be copolymerized with vinylaromatics and 1,3-dienes, e.g. ethylene, vinyl chloride, (meth)acrylic esters of alcohols having from 1 to 15 carbon atoms or vinyl esters of unbranched or branched carboxylic acids having from 1 to 15 carbon atoms, may be additionally present in copolymerized form.
If desired, from 0.05 to 10% by weight, based on the total weight of the monomer mixture, of auxiliary monomers may be additionally present in copolymerized form. Examples of auxiliary monomers are ethylenically unsaturated monocarboxylic and dicarboxylic acids, preferably acrylic acid, methacrylic acid, fumaric acid and maleic acid; ethylenically unsaturated carboxamides and nitriles, preferably acrylamide and acrylonitrile; monoesters and diesters of fumaric acid and maleic acid, e.g. the diethyl and diisopropyl esters, and also maleic anhydride, ethylenically unsaturated sulfonic acids or their salts, preferably vinylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid. Further examples are precrosslinking comonomers such as multiply ethylenically unsaturated comonomers, for example divinyl adipate, diallyl maleate, allyl methacrylate or triallyl cyanurate, or after-crosslinking comonomers, for example acrylamidoglycolic acid (AGA), methyl methylacrylamidoglycolate (MAGME), N-methylolacrylamide (NMA), N-methylolmethacrylamide, N-methylol(allyl carbamate), alkyl ethers such as the isobutoxy ethers or esters of N-methylolacrylamide, of N-methylolmethacrylamide and of N-methylol(allyl carbamate). Also suitable are epoxy-functional comonomers such as glycidyl methacrylate and glycidyl acrylate. Further examples are silicon-functional comonomers such as acryloxypropyltri(alkoxy)silanes and methacryloxypropyltri(alkoxy)silanes, vinyltrialkoxysilanes and vinylmethyldialkoxysilanes, where alkoxy groups present can be, for example, ethoxy and ethoxypropylene glycol ether radicals. Mention may also be made of monomers containing hydroxy or CO groups, for example hydroxyalkyl methacrylates and hydroxyalkyl acrylates, e.g. hydroxyethyl, hydroxypropyl or hydroxybutyl acrylate or methacrylate, and also compounds such as diacetoneacrylamide and acetylacetoxyethyl acrylate or methacrylate.
The choice of monomers or the choice of the proportions by weight of the comonomers is made so that, in general, a glass transition temperature Tg of from xe2x88x9270xc2x0 C. to +100xc2x0 C., preferably from xe2x88x9250xc2x0 C. to +50xc2x0 C., particularly preferably from xe2x88x9220xc2x0 C. to +40xc2x0 C., results. Preference is given to copolymerizing styrene and 1,3-butadiene in the ratios mentioned without further comonomers. The glass transition temperature Tg of the polymers can be determined in a known manner by means of differential scanning calorimetry (DSC). The Tg can also be calculated approximately beforehand by means of the Fox equation. According to Fox T. G., Bull. Am. Physics Soc. 1, 3, page 123 (1956): 1/Tg=x1/Tg1+x2/Tg2+ . . . +xn/Tgn, where xn is the mass fraction (% by weight/100) of the monomer n, and Tgn is the glass transition temperature in degrees Kelvin of the homopolymer of the monomer n. Tg values for homopolymers are listed in Polymer Handbook 2nd Edition, J. Wiley and Sons, New York (1975).
Suitable protective colloids are, for example, polyvinyl alcohols, polysaccharides in water-soluble form, e.g. starches (amylose and amylopectin), modified starches such as starch ethers, for example hydroxy-alkyl ether starches, dextrins and cyclodextrins, celluloses and their carboxymethyl, methyl, hydroxyethyl or hydroxypropyl derivatives, poly(meth)acrylic acid, poly(meth)acrylamide, melamine formaldehyde sulfonates and naphthalene formaldehyde sulfonates.
Preference is given to polyvinyl alcohols having a degree of hydrolysis of from 80 to 95 mol % and a Hxc3x6ppler viscosity in 4% strength aqueous solution of from 1 to 30 mPas (Hxc3x6ppler method at 20xc2x0 C., DIN 53015). Also suitable are hydrophobically modified polyvinyl alcohols having a degree of hydrolysis of from 80 to 95 mol % and a Hxc3x6ppler viscosity in 4% strength aqueous solution of from 1 to 30 mpas. Examples of such polyvinyl alcohols are partially saponified copolymers of vinyl acetate with hydrophobic comonomers such as isopropenyl acetate, vinyl pivalate, vinyl ethyl-hexanoate, vinyl esters of saturated alpha-branched monocarboxylic acids having 5 or from 9 to 11 carbon atoms, dialkyl maleates and dialkyl fumarates, e.g. diisopropyl maleate and diisopropyl fumarate, vinyl chloride, vinyl alkyl ethers such as vinyl butyl ether, olefins such as ethene and decene. The proportion of hydrophobic units is preferably from 0.1 to 10% by weight, based on the total weight of the partially saponified polyvinyl acetate, with the hydrophobically modified polyvinyl alcohols producing a surface tension of  less than 40 mN/m in 2% strength aqueous solution. It is also possible to use mixtures of the polyvinyl alcohols mentioned.
Particular preference is given to the partially saponified polyvinyl acetates comprising vinyl alcohol units and units of vinyl esters of alpha-branched carboxylic acids having 5 or from 9 to 11 carbon atoms in the specified amounts. Examples of such vinyl esters are those sold as Versatic acid vinyl esters by Shell under the trade names VeoVaR5, VeoVaR9, VeoVaR10 and VeoVaR11. Further suitable polyvinyl alcohols are partially saponified, hydrophobicized polyvinyl acetates which are obtained by polymer-analogous reaction, for example acetalization of the vinyl alcohol units by means of C1-C4-aldehydes such as butyraldehyde. The proportion of hydrophobic units is preferably from 0.1 to 10% by weight, based on the total weight of the partially saponified polyvinyl acetate. The degree of hydrolysis is from 80 to 95 mol %, preferably from 85 to 94 mol %, and the Hxc3x6ppler viscosity (DIN 53015, Hxc3x6ppler method, 4% strength aqueous solution) is from 1 to 30 mPas, preferably from 2 to 25 mPas.
The greatest preference is given to polyvinyl alcohols having a degree of hydrolysis of from 85 to 94 mol % and a Hxc3x6ppler viscosity in 4% strength aqueous solution of from 2 to 25 mpas (Hxc3x6ppler method at 20xc2x0 C., DIN 53015) and their combinations with the abovementioned hydrophobically modified polyvinyl esters in a weight ratio of from 10/1 to 1/10. The protective colloids mentioned can be obtained by means of methods known to those skilled in the art.
The protective colloid-stabilized polymer dispersions are prepared by the emulsion polymerization process, with the polymerization temperature generally being from 40xc2x0 C. to 100xc2x0 C., preferably from 60xc2x0 C. to 90xc2x0 C. In the copolymerization of gaseous comonomers such as ethylene or vinyl chloride, the polymerization can also be carried out under superatmospheric pressure, generally in the range from 5 bar to 100 bar. The polymerization is initiated using the initiators or redox initiator combinations customary for emulsion polymerization, for example hydroperoxides such as tert-butyl hydroperoxide, azo compounds such as azobisisobutyronitrile, inorganic initiators such as the sodium, potassium and ammonium salts of peroxodisulfuric acid. The initiators mentioned are generally used in an amount of from 0.05 to 3% by weight, based on the total weight of monomers. As redox initiators, use is made of combinations of the abovementioned initiators with reducing agents such as sodium sulfite, sodium hydroxymethanesulfinate or ascorbic acid. The amount of reducing agent is preferably from 0.01 to 5.0% by weight, based on the total weight of monomers.
The polymerization mixture is stabilized by means of the abovementioned protective colloids without additional emulsifiers. Preferably, some of the protective colloid is included in the initial charge and some of it is metered in after initiation of the polymerization. In general, the polymerization is carried out in the presence of from 1 to 25% by weight of protective colloid, based on the total weight of monomers. It is possible for all of the monomers to be charged initially, for all of them to be metered in or for part of them to be initially charged and the remainder metered in after initiation of the polymerization. A suitable method of preparing the polymer dispersions is described, for example, in the PCT application PCT/EP98/06102, whose disclosure in this respect is incorporated by reference into the present application.
After conclusion of the polymerization, an after-polymerization can be carried out using known methods to remove residual monomers, for example by means of after-polymerization initiated using a redox catalyst. Volatile residual monomers can also be removed by means of distillation, preferably under reduced pressure, and, if desired, by passing inert entraining gases such as air, nitrogen or steam through or over the polymerization product. The aqueous dispersions obtainable in this way generally contain from 1 to 25% by weight of protective colloid, based on the polymer, and have a solids content of from 30 to 75% by weight, preferably from 40 to 65% by weight.
To prepare polymer powders which are redispersible in water, the aqueous dispersions are dried, for example by means of fluidized-bed drying, freeze drying or spray drying. The dispersions are preferably spray dried. Spray drying is carried out in customary spray drying units, and atomization can be carried out by means of single-fluid, two-fluid or multifluid nozzles or by means of a rotating disk. The outlet temperature is generally chosen in the range from 55xc2x0 C. to 100xc2x0 C., preferably from 70xc2x0 C. to 90xc2x0 C., depending on the unit, the Tg of the resin and the desired degree of drying.
The total amount of protective colloid prior to the drying procedure is preferably at least 10% by weight, based on the polymer. To ensure redispersibility, it is generally necessary to add further protective colloids as atomization aid to the dispersion prior to drying. In general, the proportion of protective colloid prior to atomization of the dispersion is from 5 to 25% by weight, based on the polymer.
Suitable atomization aids are partially saponified polyvinyl acetates; polyvinylpyrrolidones; polysaccharides in water-soluble form, e.g. starches (amylose and amylopectin), modified starches such as starch ethers, for example hydroxyalkyl ether starches; celluloses and their carboxymethyl, methyl, hydroxyethyl, hydroxypropyl derivatives; proteins such as casein or caseinate, soya protein, gelatin; ligninosulfonates; synthetic polymers such as poly(meth)acrylic acid, copolymers of (meth)acrylates with carboxyl-functional comonomer units, poly(meth)acrylamide, polyvinylsulfonic acids and their water-soluble copolymers; melamine formaldehyde sulfonates, naphthalene formaldehyde sulfonates, styrene-maleic acid and vinyl ether-maleic acid copolymers. Preferred atomization aids are partially saponified polyvinyl acetates having a degree of hydrolysis of from 80 to 95 mol % and a Hxc3x6ppler viscosity of from 1 to 30 mPas, which may, if desired, have been hydrophobically modified as indicated above.
In the atomization, a content of up to 1.5% by weight of antifoam, based on the base polymer, has frequently been found to be useful. To prolong the shelf life by improving the caking stability, in particular in the case of powders having a low glass transition temperature, the powder obtained can be admixed with an anticaking agent, preferably in an amount of up to 30% by weight, based on the total weight of polymeric constituents. Examples of anticaking agents are calcium carbonate or magnesium carbonate, talc, gypsum, silica and silicates, preferably having particle sizes in the range from 10 nm to 10 xcexcm.
To improve the use properties, further additives can be added in the atomization. Further constituents of dispersible powder compositions present in preferred embodiments are, for example, pigments, fillers, foam stabilizers, hydrophobicizing agents.
Among the types of gypsum plaster, preference is given to xcex1- and xcex2-hemihydrate (CaSO4 1/2 H2O) in the form of, for example, builder""s plaster, stucco plaster, hard plaster of Paris or modeling plaster. However, it is also possible to modify other types of calcium sulfate, for example flooring plaster, Keene""s cement, dihydrate and anhydrite. The calcium sulfate obtained in flue gas desulfurization (FGD gypsum) is also well suited.
The gypsum plaster composition may further comprise the customary additives and modifiers. Customary additives for gypsum mortar are calcium hydroxide in an amount of preferably from 1 to 30% by weight and also inert fillers such as calcium carbonate, dolomite, clay minerals such as talc, mica, kaolin and/or quartz sand in customary amounts, preferably in amounts of from 5 to 90% by weight. The percentages by weight are in each case based on the total weight of the pulverulent gypsum plaster composition.
Modifiers which improve the processability of the calcium sulfate or calcium carbonate compositions or the properties of products manufactured therewith are, for example, salts of long-chain fatty acids such as calcium stearate, sodium oleate, silicone building protection compositions, fungicides, fibrous materials, accelerators such as dipotassium hydrogen sulfate, retarders such as tartrates, phosphates, protein derivatives, thickeners such as cellulose ethers, starch ethers, dextrins, bentonites.
The protective colloid-stabilized vinylaromatic-1,3-diene copolymer is generally used in an amount of from 0.2 to 15% by weight, based on the dry weight of the formulation. To modify the gypsum-based or CaCO3-based materials, the dispersible powder or the dispersion is mixed in suitable mixers with the calcium sulfate or the calcium carbonate and possibly further additives and modifiers and the mixture is homogenized. Preference is given to preparing a dry composition by means of dispersible powders and adding the water necessary for processing at the building site prior to processing.
The compositions modified according to the invention are particularly suitable for use as gypsum mortars for knifing fillers, joint fillers, CaSO4 flow screeds, jointing compositions, adhesive mortars or for use for producing plasterboards or plaster molds. Examples of further applications are plasters and renders or stucco work, including exterior applications. The usual applications for the correspondingly modified CaCO3-based materials are joint fillers, gypsum-free knifing fillers and plasters and renders.
It has been found that hydrophobicization according to the invention significantly improves the mechanical properties of gypsum-based or CaCO3-based materials, e.g. adhesive pull strength, flexural strength, compressive strength, abrasion resistance, water absorption, and thus the weathering resistance of knifing fillers.